1. Field of the Disclosure
This invention relates to a cooling system for cooling data centres (i.e. environments where a plurality of IT equipment, eg data servers, is operated).
2. Technical Background
By far the most popular design of existing cooling systems for data centres is the use of downflow (CRAC) units. These rely on CRAC (Computer Room Air Conditioning) units supplying cool air into a raised modular floor plenum effectively creating a higher pressure in the void than in the ICT space. This increased pressure forces cool air out of floor grilles that are generally located in what is termed a cold aisle. A cold aisle is an aisle between ICT equipment racks that has floor grilles across its width and length and has the front of two sets of racks facing one another. Cool air is drawn into the front of the equipment cabinet by the ICT equipment and discharged at the rear of the rack. The rear of the racks will generally face one another to form a ‘hot aisle’. The ‘hot aisle’ will not have any floor tiles and may have some form of baffling to assist in returning hot air back to the CRAC units to cool and re-circulate once again.
The CRAC units are generally cooled either by a chilled fluid such as water or a water/glycol mixture or by a DX (direct expansion) system utilising refrigerants such as R22, R134a, R407C or R410A. Chilled fluid systems would be connected to an external ‘chiller’. Direct expansion systems would be connected to an external means of heat rejection typically an air cooled condenser or condensing unit. Occasionally water cooled DX systems may be used in which case the CRAC unit would be connected to one or more dry air coolers (radiators) or cooling tower(s).
CRAC units currently control to return air temperature. If the return air temperature falls below set point then the cooling capacity of the unit is reduced (by staging compressors or altering coolant fluid flow rates) whilst keeping the airflow constant. This results in the supply air temperature rising and the temperature differential over the CRAC unit becoming less.
FIGS. 1 and 2 show two typical installations of downflow CRAC units. Figure one shows a typical CRAC unit located in a room with an open top return to the CRAC unit. Figure two shows a similar system but this time return air is passed via a suspended ceiling void to the top of the CRAC unit. The supply air (i.e. air supplied to cabinets) is at about 14 to 16° C. and return air at about 22 to 24° C.
The main benefits of the downflow CRAC unit method are its flexibility in terms of locating equipment racks and that redundant units can be located in the room so that if a unit needs maintaining or fails there will be sufficient ‘standby’ capacity in the room for the ICT equipment to continue functioning unaltered.
Data centres are designed to have varying grades of redundancy to ensure that the ICT equipment is continuously available to the business. The most common forms are known as N+1 and 2N where N is the total cooling required to maintain design operation of the data centre. Taking an example of each grade of redundancy;
N+1=a data centre that has a total cooling load of 200 kW that is satisfied with 4 No 50 kW CRAC units but is fitted with a fifth 50 kW unit to provide +1 redundancy therefore N (4×50 kW)+1(1×50 kW).
2N=a data centre with the same 200 kW cooling load that is satisfied with 4 No. 50 kW CRAC units but is fitted with a further 4 No 50 kW unit to provide +N redundancy therefore N (4×50 kW)+N(4×50 kW) which equates to 2N.
It is clear that N+1 is less expensive than 2N and satisfies most commercial data centre operators requirements in terms of facility resilience, this makes it the most popular option.
The disadvantages of the downflow CRAC unit method are that is cools the room, not just the ICT rack, it is not controlled at the ICT rack level and that it moves more air than is required and at temperatures that do not suit ICT equipment which will be further explained later.
An alternative existing ICT cooling solution is the use of rack coolers.
Despite some attempt at segregation hot aisle/cold aisle etc CRAC units tend to be used to cool the data centre itself and in doing so ensure that the ICT equipment is cooled. Rack coolers cool at the ICT equipment rack level.
Rack coolers generally consist of a cooling coil across which air from the rack is passed by fans. The cooling coil may be in the base of the rack and cool air be passed up the front face of the cabinet, or it may be mounted between the racks passing cold air across the front and taking the hot return air from the back of the rack. In some instances the rack is not closed to the data centre and the cooler sits on the back of the rack and cools the hot air as it leaves the rack.
Cooling mediums for these methods can be pumped refrigerant, CO2, or a chilled fluid such as water. In all methods heat rejection plant will be positioned externally to transfer the heat to atmosphere.
FIGS. 3 and 4 show two typical types of rack cooler. Figure three shows a rack cooler with the chilled water coil at its base and warm air is drawn down the back of the rack and cool air is forced up the front face of the rack. Figure four shows a similar system, this time the air is pushed horizontally across the front face of the rack and is drawn back at the rear of the rack, across the chilled water coil to begin the process once more. Supply air is typically at about 20 to 22° C. and return air at about 40 to 44° C.
The main benefits of rack coolers is that they control at a rack, not room level, and that the temperatures and airflows better match that of modern ICT equipment.
Disadvantages are that redundancy is required at the rack level which means that N+1 (the most popular configuration) is exactly the same solution as 2N. Each rack has its own cooler therefore to have N+1 each rack must have two coolers which is the same as 2N.
Another disadvantage is that the secondary cooling medium has to be run within the data centre. Some data centre operators are understandably nervous about moving large quantities of water or high pressure CO2 local to their business critical ICT equipment.
Finally, the cooling available from a rack cooler due to its different operating parameters is greater than that of the CRAC downflow system but because the rack is sealed the control loop is very tight. Some rack coolers are marketed as having up to 30 kW cooling capacity. The rack cooler is a closed system so if it were to fail there would be very little thermal inertia which results in thermal cut-out of standard ICT equipment within 5 seconds of the cooling failing.
Most if not all cooling methods of the modern data centre singularly fail to recognise that ICT equipment is perfectly capable of cooling itself as long as it is presented with the right quantity and temperature of air at its inlet and that the hot air rejected is managed away to prevent it short cycling and passing to the front of the equipment again.
The IBM blade server is an example of ICT equipment that has been developed over recent years. This server would be housed in what IBM would refer to as a Blade Center® that would comprise a plurality of these servers.
Published figures for an IBM Blade Center® (at time of writing) are 5.1 kW heat rejection based on an airflow rate of 220 l/s.
It is designed to have air introduced to its front face at between 20 and 22 degrees centigrade (° C.).
Based on the data above and using a standard equation for the calculation of nett sensible cooling we can derive that if 220 l/s of air was presented at the front face at 20° C. and it absorbed 5.1 kW of heat it would be discharged from the rear of the blade centre 19.3 degrees Kelvin (° K.) higher than it entered. So we can see that the ideal in terms of cooling for this piece of ICT equipment would be 220 l/s of air at 20° C. presented to its front face and then discharged from the rear at 39.3° C.
The table below shows the operating parameters of the two main cooling methods used and compares them with the requirements of this example of ICT equipment.
TABLE 1CriteriaExample ServerCRAC SystemRack CoolerSupply20.014.020.0temperature (° C.)Return39.024.042.0temperature (° C.)Temperature19.010.022.0differentiall/s air per kW44.083.337.8cooling
It is clear from the data above that the rack cooler method most closely matches the requirements of the server but asks the ICT equipment to have a higher temperature differential across it (due to supplying too little air) than the ideal. The CRAC system controls the room temperature and supplies air at too low a temperature which is inefficient and also supplies too much air, again this is inefficient.
FIG. 5 illustrates this imbalance. Each cabinet 7 in this example is populated with two blade systems (generating 10.2 kW). The air from the cabinets is at about 33° C. at a flow rate of about 0.44 m3/S. The maximum airflow in the cold aisle 6 (from the CRAC 1 through floor void 3 will be 2 m3/S, giving 1 m3/S per rack (in this example), i.e. about 12 kW.